In 1998, Corning sold the Pyrex brand to World Kitchen, and it appears that around then Pyrex sold in the US stopped being made out of borosilicate glass; instead, now it’s conventional soda lime glass. Borosilicate composition is the sine qua non of Pyrex; Pyrex was Pyrex — which is to say, able to go from oven to cooktop to freezer with no danger of breakage — because it was borosilicate, not normal tempered glass. Pyrex made form conventional glass just isn’t the Pyrex we all came to know in the years prior to 1998.
It’s a worse problem, actually, than the obvious, i.e. a sudden drop in quality. People buy Pyrex pan with the expectation that they can bake a chicken in it, but the new Pyrex pans just might spontaneously shatter (with some force!) under such heating. Needless to say, both Corning and World Kitchen would very much like everyone to shut up about this, but thankfully Consumer Affairs hasn’t, and won’t.
Bottom line: Do not buy Pyrex. Find a supplier who actually make borosilicate glass if you want what Pyrex used to be.